To obtain fluids, such as oil and gas, from a subterranean reservoir boreholes or wells are drilled from the surface into the reservoir. The most commonly applied method to drill a well uses a derrick or mast structure, in which a drill string is assembled and continuously extended into the borehole as the drilling progresses. Drilling is performed by rotating a drill bit attached to the end of the drill string. During the drilling process pressurized drilling fluid (commonly known as “mud” or “drilling mud”) is pumped from the surface into the hollow drill string to provide lubrication to various members of the drill string including the drill bit. On its way back to the surface through the annulus between drill string and the wall of the borehole, the drilling fluid removes the cuttings produced by the drill bit.
In most cases the pressure exerted by the drilling fluid is above the formation or pore pressure to prevent the entry of formation fluids into the wellbore during the drilling process. As a beneficial side effect, a small amount of pressurized mud enters into porous sections of the formation as it flow across those, thus leaving behind a layer of larger particles on the borehole wall. This layer is referred to as filter or mud cake. The mud cake layer prevents further fluid loss, which can be harmful, damaging formation permeability and lubricating fractures.
The barrier provided by the mud cake can potentially increase the so-called “mud window”. The mud window is a pressure range in which the driller maintains the mud pressure. The mud pressure should be sufficiently high to prevent influx from the formation whilst being low enough to prevent a fracturing of the formation and lost circulation. A wider mud window has the advantage of effectively increasing the distance that can be drilled before the open borehole requires a casing. With an increased distance between subsequent casing shoes or points, the drilling operation can be completed in a shorter time period and at reduced costs.
Considerable efforts have therefore been made to optimize the filter cake as a protective layer—mostly by adding suitable chemical compositions to the base drilling fluid in order to increase the stability of the mud cake and the adjacent formation or to increase its capability of the mud cake layer to isolate the borehole from the surrounding formation.
In the patent document SU 1361304 a bit with two off-set pairs of rollers is described for a compacting action onto the wall of a borehole. The roller are described as cylindrical rubber cased sleeves. However rubber when exposed to the hostile environment close to the drill bit exhibits a high degree of wear and tear, making the tool impractical for most applications.
In the light of the above, it is an object of the present invention to advantageously condition the interface layer between an open uncased wellbore and the surrounding formation during drilling operations.